1. Field of the Invention
The present invention relates to a crystallizing method for a semiconductor thin film, a substrate having a crystallized semiconductor thin film, a crystallizing apparatus for a semiconductor thin film, a light modulation element for use in the crystallizing apparatus, a semiconductor device formed in a crystallized thin film, and a manufacturing method for a thin-film semiconductor device, which are all applicable to an electronic apparatus such as an active-matrix flat-panel display. In particular, the present invention relates to a technology for forming an alignment mark for alignment in a case where a large-grain-sized recrystallized region is formed in an amorphous or polycrystal semiconductor thin film.
2. Description of the Related Art
A semiconductor film formation technology is an important technology for forming, on an insulating substrate, a semiconductor device such as a thin-film transistor (TFT), a contact-type sensor or a photoelectric conversion device. In usual cases, field-effect transistors with a MIS architecture, in particular, with a MOS architecture, are used as thin-film transistors.
For example, a liquid crystal display, which is an example of a flat-panel display, has such general characteristics that the display is thin in size and light in weight, the power consumption is low and color display is easily realized. By virtue of these characteristics, liquid crystal displays are widely used as displays for personal computers and other various mobile information terminals. In the case where the liquid crystal display is of an active-matrix type, thin-film transistors are used as switching devices for pixels, and as devices that constitute pixel-driving circuits.
In many cases, an active layer, that is, a carrier movement layer, of the thin-film transistor is formed of a silicon semiconductor thin film. In usual cases, the silicon semiconductor thin film is formed on an insulating substrate by, e.g. CVD or sputtering. Depending on conditions such as the temperature for film formation and the rate of film formation, the silicon semiconductor thin film is formed on the insulating substrate as amorphous silicon or polycrystalline silicon comprising many crystal grains that are divided by crystal grain boundaries.
Compared to crystalline silicon, the amorphous silicon has a carrier mobility that is lower by one or two orders of magnitude. Thus, in general, an amorphous silicon film, which is once formed, is crystallized by high-temperature heat treatment, and the crystallized silicon is used.
The carrier mobility of the polycrystal silicon, which is formed by the crystallization, is about 10 times to 100 times higher than that of amorphous silicon. This characteristic is very excellent for a semiconductor film material, of which a switching device for, e.g. a liquid crystal display is to be formed. Under the circumstances, there has recently been proposed a crystallizing method in which large-grain-sized crystallized silicon is produced in order to enhance the mobility of electrons or holes and to reduce the non-uniformity in grain boundaries in the channel region.
In recent years, applications of thin-film transistors, in which polysilicon is used as active layers, have vigorously been developed. Thin-film transistors using crystallized silicon have drawn special attention as switching devices that constitute logic circuits, such as domino logic gates and CMOS transmission gates, for their ability to execute high-speed operations. These logic circuits have been used in, for instance, driving circuits for liquid crystal displays or electroluminescent (EL) displays, multiplexers, EPROMs, EEPROMs, CCDs and RAMs.
A description will now be given of a typical prior-art process of forming a silicon thin film including a polysilicon region. In this process, an insulating substrate formed of, e.g. glass is first prepared. An SiO2 film, for instance, is then formed on the surface of the insulating substrate as an undercoat layer or a buffer layer. Subsequently, an amorphous silicon film with a thickness of about 50 nm is formed on the undercoat layer by, e.g. CVD. The amorphous silicon film is subjected to dehydrogenation treatment in order to lower the hydrogen concentration in the amorphous silicon film. The amorphous silicon film is crystallized by, e.g. an excimer laser crystallization method. Specifically, an excimer laser beam is applied to the amorphous silicon film. Thus, the amorphous silicon is melted and recrystallized into polycrystalline silicon.
At present, a silicon thin film, which is formed of the thus obtained polysilicon, is used as an active layer of an n-channel or p-channel thin-film transistor. In this case, the mobility (mobility of electrons or holes by electric field) of the thin-film transistor is about 100 to 150 cm2/V sec in the case of the n-channel type transistor, and is 80 cm2/V sec in the case of the p-channel type transistor. Using such thin-film transistors, driving circuits such as a signal line driving circuit and a scan line driving circuit may be formed on the same substrate as pixel switching devices, and thus a display in which driving circuits are integrated can be fabricated. This integral architecture can reduce the manufacturing cost of the display.
However, the electrical characteristics of the thin-film transistor, which is fabricated using the polycrystalline silicon thin film that is crystallized by the prior-art method, are not so high that the thin-film transistor can be applied to a DC converter that converts digital video data to analog video data, or to a signal processing circuit, such as a gate array, that processes digital video data. In order to apply the thin-film transistor to, e.g. a DC converter or a signal processing circuit, it is considered that a current driving performance, which is two to five times higher that that of the above-described thin-film transistor, is necessary. In addition, a field-effect mobility of about 300 cm2/V sec is necessary.
In order to achieve higher-level functions and higher added values of the display, it is necessary to further improve the electrical characteristics of the thin-film transistor. For example, in a case where a static memory that is composed of a thin-film transistor is added to each pixel in order to provide a memory function, this thin-film transistor needs to have electrical characteristics that are substantially equal to the electrical characteristics of a transistor using a single-crystal semiconductor. Thus, it is necessary to improve the characteristics of the semiconductor film.
A possible approach to improve the characteristics of a semiconductor film is to make the crystallinity of the semiconductor film closer to that of a single crystal. If the entire semiconductor film on the insulating substrate can be made into a single crystal, it becomes possible to obtain characteristics that are similar to those of a device using an SOI substrate, which has been investigated as a next-generation LSI. This attempt has been conducted for more than ten years as a 3-D device research project, but a technology for single-crystallizing the entirety of a semiconductor film has not been established. At present, there has still been a demand for formation of a semiconductor film of a single crystal on an insulating substrate.
As regards crystallization of an amorphous semiconductor film, a technique has been proposed wherein a crystallized crystal grain in the semiconductor film is grown to a larger size so that almost the same effect as in a case of substantially single-crystallizing the amorphous semiconductor film can be obtained. In this method, an excimer laser beam, which is spatially intensity-modulated with use of a phase shifter, is radiated. In this phase-modulation excimer laser crystallization method, a temperature distribution is imparted to an amorphous silicon thin film, and the amorphous silicon thin film is melted and recrystallized. Thereby, a large-grain-sized crystallized region is obtained in the crystallized silicon thin film.
In the phase-modulation excimer laser crystallization method, a phase shifter is employed to impart an intensity distribution to an excimer laser beam on the plane of the silicon thin film, a temperature gradient corresponding to the intensity distribution is provided in the silicon thin film. The temperature gradient facilitates growth of crystal silicon grains from a lower-temperature region toward a higher-temperature region in a direction parallel to the plane of the silicon thin film. As a result, compared to the conventional laser crystallization method, larger-sized crystal silicon grains can be grown.
According to this method, a crystallized silicon grain is grown to a grain size of about several microns, which can contain at least a channel region of an active device such as a thin-film transistor. By forming a thin-film transistor in the large crystal silicon grain region, it becomes possible to obtain the thin-film transistor with electrical characteristics that meet the above-described requirement.
The phase-modulation excimer laser crystallization method is an effective technique for obtaining large-grain-sized crystal silicon. However, in the phase-modulation excimer laser crystallization method, a microcrystal first grows in a low-temperature region where crystal growth beings. With this microcrystal functioning as a seed, a larger crystal grain grows. Consequently, in the recrystallized silicon thin film, there are both a region comprising a large-grain-sized crystalline silicon grain and a polycrystalline region comprising small-grain-sized crystalline silicon grains. In the case of adopting this crystallization process, it is thus necessary to exactly position a subsequently formed thin-film transistor in the region comprising the large-grain-sized crystalline silicon grain.
If the thin-film transistor is displaced from the large-grain-sized crystalline silicon region and is formed in the polycrystalline region, the electrical characteristics of the thin-film transistor would considerably deteriorate. If such a thin-film transistor with deteriorated electrical characteristics is included in, e.g. a flat-panel display, the flat-panel display would become a defective one and the manufacturing yield would considerably lower. In the case of successively forming a plurality of large-grain-sized recrystallized silicon regions in order to form a plurality of thin-film transistors on the same substrate, a sufficient positional precision cannot be obtained if the substrate is moved only by a mechanical method.
In order to solve this problem, an attempt has been made to form an alignment mark in advance in a crystallization process. Specifically, an alignment mark is formed in advance on an undercoat layer or a support substrate prior to recrystallization. A crystallizing process is executed in accordance with the alignment mark.
FIG. 9 illustrates an example of a prior-art alignment mark formation method. In order to form a silicon thin film in which a thin-film transistor (not shown) is to be formed, an insulating substrate 111 of, e.g. glass or quartz is prepared. Where necessary, an undercoat layer (not shown) of, e.g. SiO2 is formed on the insulating substrate 111. Subsequently, an alignment mark 112 of a predetermined shape is formed on the surface of the insulating substrate 111 or undercoat layer by, e.g. lithography or chemical etching. Then, for instance, an amorphous silicon thin film or polysilicon thin film 113 is formed on the support substrate. By optically recognizing the alignment mark 112, the position of the insulating substrate 111 is confirmed and a predetermined region 114 is crystallized by a phase-modulation excimer laser crystallization method.
The crystallized region 114 includes both a polycrystalline region 116 comprising small-grain-sized silicon grains, from which crystallization has begun, and a large-grain-sized crystal region 115 comprising a large-grain-sized single crystal that has been grown from the small-grain-sized silicon grain. Crystallization is performed with reference to the alignment mark 112, and the polycrystalline region 116, which is the crystallization start region, and the large-grain-sized crystal region 115 are disposed at predetermined positions on the insulating substrate.
The above-described prior-art method, however, requires an additional step of forming the alignment mark 112 by, e.g. lithography. Thus, such a problem arises that the manufacturing process becomes longer.
On the other hand, an active layer (carrier movement layer) of the thin-film transistor is formed of, e.g. a silicon semiconductor thin film. Silicon semiconductor films are generally classified into amorphous silicon (a-Si) films and polycrystalline (non-single-crystal crystalline silicon) films.
The polycrystalline silicon is mainly polysilicon (poly-Si), but microcrystal silicon (μc-Si) is also known as polycrystalline silicon. Usable semiconductor film materials other than silicon include, for instance, SiGe, SiO, CdSe, Te and CdS.
As a crystallization method for increasing the crystal grain size of crystallized silicon, there has been proposed a method in which light with a specified intensity distribution, which is produced by interposing a phase shifter (phase modulation element) in an optical system, is radiated on a semiconductor film that is an object of processing (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2000-306859 (hereinafter referred to as Document 1)).
In addition, as a method of increasing the grain size of a crystal grain in crystallizing an amorphous semiconductor thin film, it has been proposed by the inventors of the present invention to radiate an excimer laser beam that is spatially intensity-modulated by using a phase modulation element. In this method, an amorphous silicon thin film is melted and recrystallized by a laser beam, and thus made into a polysilicon thin film. This method is called “phase-modulation excimer laser crystallization method” (see, e.g. Surface Science, Vol. 21, No. 5, pp. 278-287, 2000 (hereinafter referred to as Document 2)). The inventors have been advancing development for industrializing this technology.
A large-grain-sized single-crystal silicon grain, which is formed by the crystallization method of Document 2, is surrounded by countless small-grain-sized polycrystalline silicon grains or by amorphous silicon. The large-grain-sized single-crystallized silicon grain has such a size as to permit formation of a channel region or channel regions of one or more thin-film transistors.
The inventors have been advancing development for industrializing, e.g. the crystallization method disclosed in Document 2. Even if a large-grain-sized silicon grain is successfully obtained by this development, there arises such a case that a channel region of a thin-film transistor is not formed within the region of the large-grain-sized single-crystal silicon grain. If the positional displacement occurs, such a problem arises that the electrical characteristics of the thin-film transistor, for example, switching characteristics, would extremely deteriorate.
It is thus imperative to make the large-grain-sized single-crystal silicon grain, which is obtained by the above-described crystallization, agree with the channel region of the thin-film transistor in order to achieve high-speed switching characteristics of the thin-film transistor. In order to make the large-grain-sized single-crystal silicon grain agree with the position of formation of the thin-film transistor, when crystallization is performed by the phase-modulation method, the inventors have been studying simultaneous formation of an alignment mark for alignment.
In the method of performing the crystallization by the phase-modulation method, a high fluence is required. However, with such a high fluence, ablation (film destruction) occurs in a region that does not undergo phase modulation. Thus, there is such a problem that formation of a crystal grain and formation of an alignment mark cannot be performed at the same time.
A method and an apparatus for making the range of the large-grain-sized single-crystal silicon grain, which is formed on the substrate, exactly agree with the position, where the thin-film transistor is to be formed, have not been established.
The present invention can provide a method and an apparatus, which are capable of exactly forming a semiconductor active device, that is, a switching device, in a region where a large-grain-sized crystal grain is formed by crystallization, by virtue of the formation of an alignment mark that characterizes the present invention.